We have devised a method to select galaxies that are isolated in their dark matter halo ( N = 1 systems ) and galaxies that reside in a group of exactly two ( N = 2 systems ) .
Our N = 2 systems are widely-separated ( up to \sim 200 h ^ { -1 } kpc ) , where close galaxy-galaxy interactions are not dominant .
We apply our selection criteria to two volume-limited samples of galaxies from SDSS DR6 with M _ { r } -5 \log _ { 10 } h \leq -19 and -20 to study the effects of the environment of very sparse groups on galaxy colour .
For satellite galaxies in a group of two , we find a red excess attributed to star formation quenching of 0.15 \pm 0.01 and 0.14 \pm 0.01 for the -19 and -20 samples , respectively , relative to isolated galaxies of the same stellar mass .
Assuming N = 1 systems are the progenitors of N = 2 systems , an immediate-rapid star formation quenching scenario is inconsistent with these observations .
A delayed-then-rapid star formation quenching scenario with a delay time of 3.3 and 3.7 Gyr for the -19 and -20 samples , respectively , yields a red excess prediction in agreement with the observations .
The observations also reveal that central galaxies in a group of two have a slight blue excess of 0.06 \pm 0.02 and 0.02 \pm 0.01 for the -19 and -20 samples , respectively , relative to N = 1 populations of the same stellar mass .
Our results demonstrate that even the environment of very sparse groups of luminous galaxies influence galaxy evolution and in-depth studies of these simple systems are an essential step towards understanding galaxy evolution in general .